1. Field of the Invention
The present invention relates to a semiconductor device. In this specification, a semiconductor device refers to a semiconductor element itself or a device including a semiconductor element. As such a semiconductor element, a thin film transistor is given, for example. Therefore, the semiconductor device includes a liquid crystal display device, a memory device, and the like.
2. Description of the Related Art
In recent years, a portable semiconductor device such as a mobile computer has been rapidly spread. Although such a portable semiconductor device includes a power storage portion and operates using power fed from the power storage portion, the charge/discharge capacity of the power storage portion is finite. Accordingly, there is a demand for reducing power consumption as much as possible.
However, power consumption of a processor has been increased with an improvement in performance of a semiconductor device. In view of this situation, reduction in power consumption of a processor has been attempted using a power gating technique in which power consumption is reduced by stopping power feeding to a logic circuit which is not in processing (see Patent Document 1).
When a logic circuit to which power feeding is stopped and a logic circuit to which power feeding is being performed are electrically connected to each other by a signal line in the case of using the power gating technique for the processor, reverse current to a power supply line or an increase in consumption current due to an intermediate potential may be caused. Such a problem can be solved by provision of an isolator circuit between the logic circuit and the signal line.